Field of the Invention
The present invention relates to fluid property measuring transducers and methods and, more particularly, to electromechanical transducers and associated methods that can determine the viscosity of a fluid.
Description of the Related Art
Viscosity is one of the most sensitive and essential characteristics that determines a fluid's quality. It is important in many industrial and scientific applications. The transducers used for measuring viscosity are classified based on how the flow is initiated or maintained. Common types include: Rotational, Falling object, Capillary, Vibration and Ultrasonic, with the later two induce oscillatory motions in the fluid. Vibrational transducer has certain advantages that are used widely as process viscometers. Vibrational transducers are operated by measuring the damping of an oscillating resonator immersed in a testing fluid. The resonator is excited into either torsional or linear oscillation via means of electromeganetic induction or piezoelectric effect. Resonators that employ linear vibrating beams are described by J. G. Woodward in “A Vibrating Plate Viscometer”, Journal of Colloid Science, 1953 followed by his own U.S. Pat. No. 2,696,735 and also by U.S. Pat. No. 5,067,344 where a vibrating bar with a tip blade is used to generate shear oscillation of the fluid. A tuning fork transducer described in U.S. Pat. No. 4,729,237 employs a pair of vibrating beams with integrated sensor plates as resonators. The vibrating beams are driven in anti-phase as in a tuning fork and a separate detector is used to measure the amplitudes. In the described arts, the driver and pick up transducers are separated from the resonators which require additional means of support and operate the transducers.
U.S. Pat. No. 6,044,694 describes an integrated piezoelectric bender type transducer for measuring viscosity and density of the fluid, which has the merit of small footprint, simple construction and low cost. However the vibration of the bender not only excites shear wave but also appreciable amount of compression wave in the fluid. The resonant frequency change and the damping of the transducer are influenced by both the density and viscosity of the fluid. The detailed analysis is described by W. Y. Shih et. al in “Simultaneous liquid viscosity and density determination with piezoelectric uniform cantilevers”, Journal of Applied Physics, 2001. In the described art, the determination of viscosity and density from damping and resonant frequency separately is based on an oscillating sphere model and is applicable to a limited range of viscosities.
A novel type of vibrational electromechanical transducer and the measurement principle are disclosed in the current invention that will address some of the aforementioned problems. The transducer comprises of composite beam and sensor plate, while the composite beam has integrated electromechanical transduction materials for driving and sensing the transducer response. The sensor plate is attached at the amplitude anti-node of the composite beam for optimization of the transducer's response to fluid viscosity. The sensor plate is orthogonal to the composite beams such that when the composite beam is excited into bending vibration, the motion of the sensor plate is along its plane. A pure shear oscillation of the fluid can be excited around the sensor plate. Although density influence on damping of the transducer is not completely eliminated, it can be accurately predicted. The transducer's resonant frequency, amplitude or electrical conductance can be measured to obtain a response function. The response function has proved relation to the viscosity and density of the fluid. The current invention also provides several means of excitation and sensing that optimize the response of the transducer to fluid viscosity. The advantages of the current transducer owns to its simple, robust, cost-effective construction, ease of measurement and yet with improved accuracy.